Thread revival time!
I have had a similar problem:
I cracked the screen on my Nexus 4 & after fitting a replacement, all was good except the screen would go blank on making a call.
Temp fix was to have on/off button end calls.
After much research on the net, I eventually replaced the proximity sensor (I don't have a screen protector, BTW). The screen would then work as it should, but in an intermittent fashion.
After disassembling for the umpteenth time, & studying the area of the sensor at length, I ended up placing a small packing cut from an old sim card, directly on top of the proximity sensor, as I had observed that it seemed to sit up & therefore slightly away from the internal screen, when I had it apart.
That has solved the problem, it now works reliably every time.
Hope this helps some others!